Plasma indicators of muscle cell leakage and of hemolysis were studied in 23 runners before and after a marathon race. Blood samples were drawn from an antecubital vein the morning before the race (baseline), at 3 p.m., i.e., 2 h before the start, on arrival, 12 and 36 h, and 7 days later. Compared with the baseline values, the plasma creatinine phosphokinase MM and MB subfractions, aldolase and glutamicoxaloacetic transaminase activity were increased immediately after the race, rose further 12 h after the marathon, and remained elevated the race, rose further 12 h after the marathon, and remained elevated 36 h and 7 days later. The plasma lactate dehydrogenase activity and myoglobin concentration were increased on arrival and returned to the pre-race activity 7 days after the marathon. Compared with the pre-race values, the plasma haptoglobin concentration was decreased immediately and 12 h after the marathon. Our data show that indicators of muscle cell leakage and of hemolysis in plasma, withdrawn after a marathon race, remained elevated for up to 7 days after the race.